The usefulness of the phenomenon of magnetostrictivity in linear displacement or position measuring devices is recognized by the prior art; for example, see Redding, U.S. Pat. No. 4,305,283; McCrea et al, U.S. Pat. No. 4,158,964; Krisst, U.S. Pat. No. 4,071,818; Edwards, U.S. Pat. No. 4,028,619; and Tellerman, U.S. Pat. No. 3,898,555. Common to several of these devices are a magnetostrictive wire which runs in a straight line path through the measurement field, means for inducing a torsional strain at a given position along the wire, and a magnet displaceable along the wire, either by connection to a movable body such as a machine tool or by reason of association with a float device. Generally speaking, the position of the magnet represents the position of the monitored quantity and is determined as a function of the time required for a torsional ultrasonic disturbance to propagate from one end of the wire through the area of influence of the magnet or from the position of the magnet to a sensing apparatus located at one end of the wire.
There is a problem with the devices of the prior art. Accurate displacement measurement depends upon accurate determination of the time of arrival of the magnetostrictive response. Whether the magnetostrictive wire is electrically excited and the mechanical response detected or vice versa, the magnetostrictive response is generally in the form of a damped sine wave. Accurate determination of a reliable position within such a damped sine wave is a difficult problem.
U.S. Pat. No. 4,943,773 issued Jul. 24, 1990 to Koski et al proposes a solution to this problem. This patent proposes to make the time determination based upon the first zero crossing after the signal passes a predetermined threshold value. This technique is generally acceptable but in many instances false signals can be caused by noise.
Therefore there is a need in the art for a damped sine wave detection technique which reliably finds the same point within the signal. This would be particularly useful in magnetostrictive displacement transducers.